An antenna transduces electromagnetic waves between a guided mode and unguided radiation. The use of antennas for transmission and reception of such radiation antedates a full understanding of the properties of antennas. Consequently, many of the terms used in the antenna arts have meanings which, while well understood in those arts, may be confusing to the less skilled. For example, the term “antenna beam” is ordinarily understood to refer to the unguided radiation emitted from an antenna when guided waves are applied to its “feed” point or port. However, the term also applies to the response of an antenna to received unguided radiation as manifested by guided electromagnetic waves at the “feed” in the presence of plane wave unguided radiation, and the characteristics of the antenna beam in a reception mode are identical to the characteristics of the beam in a transmission mode. The “feed” port may receive guided radiation from an external source when the antenna is operated in a transmitting mode, and may also generate or produce guided waves when the antenna receives unguided radiation. Thus, the transmission and reception of electromagnetic signals are conceptually linked, and the term “transmission/reception” can be applied to the antenna function.
An antenna “beam” may be characterized in a simplistic manner by specifying the solid or subtended angle, as seen from the antenna, in which the beam resides. The subtended angle is determined at a given relative power level, such as −3 dB, relative to the peak power level of the beam. The measurement of directivity of an antenna is made by comparing the “strength” of the radiation at the (or a) peak of the beam with the strength which the radiation would have if it were uniformly distributed over a sphere (over all solid angles). Antenna directivity is a theoretical construct, which is often used interchangeably with antenna “gain.” The gain of an antenna is a combination of the directivity together with the heating or dissipative (and possibly other) losses associated with the antenna, and thus is something which can be measured. The determination of gain is generally made by comparing the measured energy at beam peak with the beam-peak energy of a standard antenna, such as a monopole, dipole, or simple horn. Explanations of antenna operation may be couched in terms of either transmission or reception, depending on which is easier to understand. However, it should be understood that an equivalent explanation applies to operation in the other mode.
Many modern antennas for electromagnetic communication or surveillance uses require substantial or “high” directivity, or the ability to form a radiated beam which subtends a relatively small angle. This is associated with high gain. High gain is desirable in order to place maximum electromagnetic signal energy at the reception point, or equivalently for extracting the maximum amount of guided wave energy from a received unguided wave. The attaining of high directivity or high gain is ordinarily associated with a large “radiating aperture,” which relates to the physical dimensions of the antenna in a plane generally orthogonal to the direction of electromagnetic wave propagation or radiation. In the past, “reflector” antennas have been used to attain relatively large apertures. Everyone has seen at least pictures of terrestrial “dish” antennas used for space communications. Such antennas attain a large radiating aperture by intercepting unguided waves over a relatively large area, and “focussing” the radiation to a smaller area, where the antenna proper (as distinguished from the reflector) is located. The antenna proper, located at the focus of the reflector, has more electromagnetic energy available to transduce into guided-wave form for use by a receiving apparatus than it would have without the reflector.
Modern communication or radar systems achieve many advantages, including inertia-free scanning, by the use of an array antenna occupying the radiating aperture. An array antenna often includes at least a line array, and often a two-dimensional array, of antenna elements, which are “fed” from a common source by means of adjustable phase shifters, and possibly adjustable attenuators, to enable the antenna beam to be moved or directed in space without the need to physically move the antenna itself. In many situations, the antenna array is a two-dimensional array of elemental antennas, each of which elemental antennas is fed (in either the transmission or reception mode) with electromagnetic signals having phase and or amplitude which differ from one antenna element to the next (or from one group of antenna elements to the next). The structure which provides the desired phase shifts and or amplitude adjustment is known as a “beamformer.”
The manufacture of array antennas is well known in the art. In the design of array antennas, the spacing of the elemental antennas is often selected in conjunction with the desired operating wavelength to mitigate certain unwanted “grating” antenna lobes. In general, the spacing between adjacent elemental antennas in an array is maintained at one-half wavelength or less, although some antennas take advantage of the grating lobes in producing their desired beam shapes.
Among the problems associated with the manufacture of array antennas is the need to associate with each elemental antenna (or group of elemental antennas) a beam control element, such as a phase shifter, an attenuator, or both. At the frequencies at which many array antennas operate, signal transmission path lengths must be minimized, in order to avoid unwanted losses in the transmission paths. Consequently, the control elements must be kept close to the associated antenna elements. A common arrangement is to physically place the control element immediately behind its associated elemental antenna, where the radiation side of the radiating aperture is the corresponding “front.” Such an arrangement is described in U.S. Pat. No. 5,459,474, issued Oct. 17, 1995 in the name of Mattioli et al. The Mattioli et al. arrangement includes an array of horn-like elemental antennas fabricated in a conductive plate, with a slide-in carrier which mates with the elemental antennas. The resulting structure is complex and expensive. A short-horn antenna suitable for such use is described in U.S. Pat. No. 5,359,339, issued Oct. 25, 1994 in the name of Agrawal et al.
Other patents describe various prior approaches to making mating connections between antenna elements and a beamformer. U.S. Pat. No. 5,898,409, issued Apr. 27, 1999 in the name of Holzman describes an elemental antenna adapted for use in an antenna array. U.S. Pat. Nos. 6,081,988 and 6,081,099, both issued Jul. 4, 2000 in the name of Pluymers et al., describe interconnection of a planar circuit to other circuits, such as beamformers, by way of compliant fuzz buttons in a coaxial transmission-line structure. U.S. Pat. No. 6,316,719, issued Nov. 18, 2001, and U.S. Pat. No. 6,031,188, issued Feb. 29, 2000, both in the name of Pluymers et al. describe the use of compliant “fuzz buttons” in a transmission line for use in coupling together planar circuits. U.S. Pat. No. 6,590,478, issued Jul. 8, 2003 in the name of Pluymers describes a coaxial connector made from spring material for providing electromagnetic coupling between mutually parallel printed-circuit boards. U.S. Pat. No. 6,465,730, issued Oct. 15, 2002 in the name of Pluymers et al. describes fabrication of a circuit module with a coaxial transmission line for facilitating connections of a module to a “radio frequency” (RF) manifold, such as a beamformer. Many of the techniques described in these patents require substantial labor for making the large numbers of interconnections between the beamformer or control structure and the array of antenna elements.
Improved or alternative arrangements or methods are desired for array antennas.